Implement :header command

Usage: *header* [-f] <field> [value]
	Add a new email header. If the header already exists, -f must be
	specified to replace the given value.

commands/compose/header.go

@@ -0,0 +1,74 @@
+package compose
+
+import (
+	"fmt"
+	"strings"
+
+	"git.sr.ht/~sircmpwn/aerc/commands"
+	"git.sr.ht/~sircmpwn/aerc/widgets"
+	"git.sr.ht/~sircmpwn/getopt"
+)
+
+type Header struct{}
+
+var (
+	headers = []string{
+		"From",
+		"To",
+		"Cc",
+		"Bcc",
+		"Subject",
+		"Comments",
+		"Keywords",
+	}
+)
+
+func init() {
+	register(Header{})
+}
+
+func (Header) Aliases() []string {
+	return []string{"header"}
+}
+
+func (Header) Complete(aerc *widgets.Aerc, args []string) []string {
+	return commands.CompletionFromList(headers, args)
+}
+
+func (Header) Execute(aerc *widgets.Aerc, args []string) error {
+	if len(args) < 2 {
+		return fmt.Errorf("Usage: %s [-f] field [value]", args[0])
+	}
+
+	opts, optind, err := getopt.Getopts(args, "f")
+	if err != nil {
+		return err
+	}
+	var (
+		force bool = false
+	)
+	for _, opt := range opts {
+		switch opt.Option {
+		case 'f':
+			force = true
+		}
+	}
+
+	composer, _ := aerc.SelectedTab().(*widgets.Composer)
+
+	if !force {
+		headers, _, err := composer.PrepareHeader()
+		if err != nil {
+			return err
+		}
+
+		if headers.Has(strings.Title(args[optind])) {
+			return fmt.Errorf("Header %s already exists", args[optind])
+		}
+	}
+
+	composer.AddEditor(strings.Title(args[optind]),
+		strings.Join(args[optind+1:], " "), false)
+
+	return nil
+}

doc/aerc.1.scd

@@ -318,6 +318,10 @@ message list, the message in the message viewer, etc).
 	configuration. For details on configuring outgoing mail delivery consult
 	*aerc-config*(5).
 
+*header* [-f] <field> [value]
+	Add a new email header. If the header already exists, -f must be
+	specified to replace the given value.
+
 *toggle-headers*
 	Toggles the visibility of the message headers.